Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 301155

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
655 AM CDT TUE AUG 30 2016

.SHORT TERM... (Through Wednesday Night)
Issued at 251 AM CDT Tue Aug 30 2016

A pattern change will begin to unfold today. 06z satellite
imagery depicted a vort max moving into northwestern Ontario, and
this feature was well-handled by most models. As this feature
continues sliding southeastward, its upper trough axis will merge
with a separate disturbance noted over CO/NEB early this morning,
and this upper support eventually drives a cold front through the

The sfc front will arrive in two stages: first the wind shift
line today/tonight, then the moisture gradient tomorrow/tomorrow
night. Both stages will be accompanied by increased coverage of
SHRA/TSRA, partly due to sfc convergence and partly due to
increased upper support. After the moisture gradient has moved
through the area, a cooler and drier air mass will then settle
across the region for the end of the week.

Convergence along the wind shift line was partially responsible
for the clusters of SHRA/TSRA which developed early this morning
from KS through MO into IA. Scattered SHRA/TSRA will accompany the
wind shift line as it continues sagging southward today/tonight.
As mentioned above, an elongated PV anomaly was noted across
CO/NEB, and a slight eastward trend to this feature means that
increased upper support will be available today over part of the
CWA (mainly northern MO) compared to previous days. PoPs have been
adjusted to reflect higher chances farther north.

The moisture gradient is fcst to approach the MO/IA border by 12z
Wed. Although limited upper support combined with daytime heating
ahead of the bdry may initially slow its southward progress, the
arrival of the aforementioned upper trough after 18z will finally
push the bdry southward through the CWA. The combination of a
moist air mass, a sfc bdry to focus the convection, and upper
support should produce at least scattered SHRA/TSRA near and ahead
of the bdry. Locally heavy rainfall will be possible given that PW
values are forecast to be near 2", which is roughly +2 SD for
this time of year.


.LONG TERM... (Thursday through Monday)
Issued at 251 AM CDT Tue Aug 30 2016

Cooler and drier conditions will settle across the region for
Thu/Fri. Friday in particular looks spectacular attm. Thursday
will be the first full day for the CWA within the post-frontal air
mass, and a sfc high pressure center building into the Great Lakes
will ensure dry easterly/northeasterly flow continues for Fri. The
drier air mass and limited cloud cover will promote good
radiational cooling conditions for both Thu and Fri nights, and
lows in the 50s to 60s still look reasonable. The surface high
begins moving off to the east over the weekend, leading to a
slight warm-up for Saturday and then a more noticeable warmup for
Sunday once return flow begins in earnest. Precip chances also
begin to increase late this weekend when the upper pattern shifts
to southwest flow aloft.



.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 628 AM CDT Tue Aug 30 2016

Active fcst prd with a cold front and assoc convection and mrng
fog. The IFR fog should dsspt by 14Z. The front is fcst to move
thru the terminals this evng and tonight. There is ongoing
convection across NW MO...sthrn IA and nthrn IL in assoc with the
bndry. This activity is expected to continue to percolate thru the
day as it slowly sags S in advance of the cold front. Have a prd
of precip at KUIN for this aftn tied to timing of the bndry and
best diurnal heating...but SHRAs/TSTMs could impact the terminal
this mrng too as they are not too far away as of TAF issuance.
Have VCTS at KCOU for this aftn...but could be precip there this
mrng too as have noticed ISLD SHRAs dvlpng just W of the terminal
over the past hour. Not confident enough in coverage...placement
or timing at STL metro terminals to include any mention attm...but
there certainly could be precip at those sites at just about any
point today as well. Coverage of precip should diminish this evng
but there will still be scttrd convection thru the night as has
been the case along the bndry for the past several nights. Winds
actually come around to the N this aftn in advance of the actual
cold front and remain light and nthrly overnight...though may end
up bcmng VRB or calm.


Chances for convection increase this aftn as a cold front
approaches. Not confident enough in coverage or timing to include
mention attm...but could be precip at just about any time thru
the fcst prd. Precip chances diminish this evng but there will
still be at least scttrd convection across the area tonight that
may or may not impact the terminal. Precip chances ramp up again
tomorrow as secondary cold front approaches.





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